The present invention relates generally to closely coupled gas atomization of metals. More particularly, it relates to methods of operation of close-coupled atomization systems and for preparing metal powders which result in increased yields of fine particles. Most particularly, it relates to methods for positioning the melt stream flow away from the atomization plume center toward the atomization plume periphery resulting in the efficient atomization of metals, specifically superalloys.
The development of atomization systems having fluid, such as gas, atomization nozzles for the production of metallic powders started with remote gas jets, or metal freefall designs, and more recently evolved to close-coupled designs in the quest for greater efficiency and increased yields of fine powder. Many of the early remote jet designs employed a small number of individual gas jets. As the designs matured, the number of jets increased until the limiting case of an annular jet was employed. Almost universally, (see U.S. Pat. No. 4,401,609), the technology moved toward the application of axisymmetric melt and axisymmetric gas flows for fine powder efficiency improvements. The knowledge base regarding axisymmetric melt and axisymmetric gas flows generated with remote gas jets was carried over into the design of early close-coupled nozzle atomization systems. During early efforts to increase fine powder yields, gas plenum designs received considerable attention in order to ensure a high degree of gas flow symmetry. For a detailed discussion of the history of the atomization of melts, both axisymmetric and asymmetric (non-axisymmetric), see "Atomization of Melts for Powder Production and Spray Deposition," A. J. Yule and J. J. Dunkley, Oxford University Press, 1994, the disclosure of which is hereby incorporated by reference.
Conventional close-coupled atomization gas nozzles and melt guide tubes typically include axisymmetric melt guide tubes with either annular gas nozzle orifices or multiple discrete gas jets. Although multiple jet designs represented a deviation from purely axisymmetric atomization, there is significant evidence that the individual gas jet streams merged together providing a substantially axisymmetric gas flow prior to contacting the liquid melt stream.
While close-coupled or closely coupled metal atomization is a relatively new technology, methods and apparatus for the prior practice of close-coupled atomization are set forth in commonly owned U.S. Pat. Nos. 4,619,597; 4,631,013; 4,801,412; 4,946,082; 4,966,201; 4,978,039; 4,993,607; 5,004,629; 5,011,049; 5,022,150; 5,048,732; 5,244,369; 5,289,975; 5,310,165; 5,325,727; 5,346,530 and 5,366,204, the disclosures of each are incorporated herein by reference. Among other things, these patents disclose the concept of close coupling, i.e., to create a close spatial relationship between the point at which a melt stream emerges from a melt guide tube orifice and a point at which a gas stream emerges from a gas nozzle orifice to impact or intersect the melt stream and interact therewith to produce an atomization zone.
Because known prior attempts to operate closely coupled atomization apparatus resulted in many failures due to the many problems which were encountered, most of the prior art, other than those mentioned above, for atomization technology concerned remotely coupled apparatus and practices, the technology disclosed by the above referenced patents is believed to be one of the first, if not the first, successful closely coupled atomization systems to be developed that had potential for commercial operation.
For a metal atomization processing system, accordingly, the higher the percentage of the finer particles which are produced the more desirable the properties of the articles formed from such fine powder by conventional powder metallurgical techniques. For these reasons, there is a strong economic incentive to produce higher and higher yields of finer particles through atomization processing.
As pointed out in the commonly owned patents above, the close-coupled atomization technique results in the production of powders from metals with a higher concentration of fine powder. For example, it was pointed out therein that by the remotely coupled technology only about 3% of powder produced industrially is smaller than 10 microns and the cost of such powder is accordingly very high. Fine powders of less than 37 microns in diameter of certain metals are used, for example, in low pressure plasma spray applications. In preparing such fine powders by remotely coupled techniques, as much as about 60% to about 75% of the resulting powder must be scrapped because it is oversized. The need to selectively separate out and keep only the finer powder and to scrap the oversized powder increases the cost of producing usable fine powder.
Further, the production of fine powder is influenced by the surface tension of the melt from which the fine powder is produced. High surface tension melts increase the difficulty in producing the fine powder and, thus, consume more gas and energy. The remotely coupled industrial processes for atomizing powder of less than 37 microns average diameter from molten metals having high surface tensions have yields on the order of about 25 weight % to about 40 weight %.
A major cost component of fine powder prepared by atomization and useful in industrial applications is the cost of the gas used in the atomization. The gas consumed in producing powder, particularly the inert gas such as, for example, argon, is expensive. Thus, it is economically desirable to be able to produce a higher percentage of fine powder particles using the same amount of gas.
As is explained more fully in the commonly owned patents referred to above, the use of the close-coupled atomization technology resulted in the formation of higher concentrations of finer particles than was available through the use of prior remotely coupled atomization techniques.
With rare exception, for both close-coupled and remote atomization systems, designers have attempted to maintain an axisymmetric relationship between the melt flow and the gas flow. Most often, this was accomplished by using a circular melt stream surrounded by an annular, circular gas jet or a circular array of individual gas jets. Some linear atomizers have been reported using a long thin rectangular slit for the melt orifice (see U.S. Pat. No. 4,401,609). But even here the gas jet geometry is designed to provide a uniform melt spray pattern along the long axis of the slit. Only one remote atomizing nozzle and one, non-axisymmetric close-coupled atomizing nozzle are known to have existed prior to the non-axisymmetric system disclosed herein (see U.S. Pat. Nos. 4,631,013 and 4,485,834). Few, if any, non-axisymmetric melt guide tube exit orifices or non-axisymmetric gas orifice configurations are believed to have been proposed in order to achieve higher yields of fine particles.
While the early close-coupled atomization systems and methods increased the yields of fine powder relative to the metal free fall remotely coupled system, there is a continuing industrial demand for additional increased yields of ultra fine metal powders, e.g., powders having a particle diameter smaller than 37 microns. Accordingly, there is a need to develop metal atomization methods which can increase the yield of such ultra fine powder and narrow the distribution of particle sizes formed and do so with increased efficiency and lower cost. Any resulting methods should produce improved fine powder yields while being compatible with at least one and preferably both low and high melt superheat metal processing systems.